iommu/arm-smmu-v3: Acknowledge pri/event queue overflow if any

When an overflow occurs in the PRI queue, the SMMU toggles the overflow
flag in the PROD register. To exit the overflow condition, the PRI thread
is supposed to acknowledge it by toggling this flag in the CONS register.
Unacknowledged overflow causes the queue to stop adding anything new.

Currently, the priq thread always writes the CONS register back to the
SMMU after clearing the queue.

The writeback is not necessary if the OVFLG in the PROD register has not
been changed, no overflow has occured.

This commit checks the difference of the overflow flag between CONS and
PROD register. If it's different, toggles the OVACKFLG flag in the CONS
register and write it to the SMMU.

The situation is similar for the event queue.
The acknowledge register is also toggled after clearing the event
queue but never propagated to the hardware. This would only be done the
next time when executing evtq thread.

Unacknowledged event queue overflow doesn't affect the event
queue, because the SMMU still adds elements to that queue when the
overflow condition is active.
But it feel nicer to keep SMMU in sync when possible, so use the same
way here as well.

Signed-off-by: Tomas Krcka <krckatom@amazon.de>
Link: https://lore.kernel.org/r/20230329123420.34641-1-tomas.krcka@gmail.com
Signed-off-by: Will Deacon <will@kernel.org>

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